This invention relates to polymer sheet and, more particularly, to polymer sheet comprising polyvinyl butyral featuring enhanced blocking resistance accomplished by a process involving incorporation of certain fatty acid amides as additives to the polymeric sheet.
Plasticized polyvinyl butyral (hereinafter xe2x80x9cPVBxe2x80x9d) is commonly used in the manufacture of polymer sheets for use as interlayers in light transmitting laminates such as safety glass or polymeric laminates. Safety glass typically refers to a transparent laminate comprised of a polyvinyl butyral sheet sandwiched between two panes of glass. Safety glass often is used to provide a transparent barrier in architectural and automotive openings. Its main function is to absorb energy, such as caused by a blow from an object, without allowing penetration through the opening, thus minimizing damage or injury to the objects or persons within an enclosed area. Additives to the sheet formulation generally include adhesion control agents (xe2x80x9cACA""sxe2x80x9d) to modify adhesion of the sheet to the glass so that a suitable level of adhesion can be maintained so as to prevent spalling of the glass and yet provide adequate energy absorption on impact. The interlayer sheet also may be modified to impart additional beneficial effects to safety glass such as to attenuate acoustic noise, reduce UV and/or IR light transmission, and/or enhance the aesthetic appeal of window openings.
Safety glass typically is formed by a process in which two layers of glass and a plastic interlayer, such as PVB, are assembled into a pre-press, tacked into a pre-laminate, and finished into an optically clear laminate. The assembly phase involves laying down a piece of glass, overlaying a PVB sheet, laying down a second piece of glass, and then trimming the excess PVB to the edges of the glass layers.
The plastic interlayer generally is produced by mixing PVB polymer with one or more plasticizers, and optionally with one or more other ingredients, and melt processing the mix into sheeting, which typically is collected and rolled for storage and transportation. In the lamination process for automotive windscreens, sections of PVB sheet typically are cut from the roll and these cut sections are shaped and/or stacked for assembly. A cut section then is taken from the stack and assembled in layered arrangement with a rigid substrate (e.g., glass sheet having a particular optical quality) such that a face of the rigid substrate and a face of the cut section are disposed in intimate contact and form a pre-press laminate assembly. Alternatively, this laminate assembly may be formed by interleaving multiple cut section(s) with multiple rigid sheets.
Plasticized PVB sheet, whether in roll form or in stacked form, inherently tends to stick to itself (xe2x80x9cblockxe2x80x9d) at ambient temperatures typically encountered before and during the laminating process. Many attempts to enhance the blocking resistance of PVB have been made, including mechanical roughing of the sheet surfaces (e.g., embossing), applying a powder such as sodium bicarbonate to the facing contacting sheet surfaces, and chemically treating the surfaces of the PVB sheeting with acids. Such surface treatments unfortunately often introduce undesireable handling or glass adhesion issues. In another common practice to avoid such blocking, the PVB sheeting may interleaved with another sheet material, such as polyethylene, or may be stored and transported under refrigeration, e.g., at temperatures from about 5 to about 15xc2x0 C. However, for variations of standard PVB sheeting, such as PVB sheeting incorporating high plasticizer content to accomplish an enhanced noise reduction (acoustic) function, blocking may occur even under refrigeration conditions.
Additionally, it has been proposed to incorporate various blocking-resistant materials into the PVB. Examples of such materials include montanic acid ester, ethylene glycol or propylene glycol C1 to C25 alkylethers, e.g., diethylene glycol dodecyl ether and tripropylene glycol dodecyl ether. Incorporation of such materials into the PVB, however, may adversely affect the optical properties of the resulting laminate or the adhesive properties of the PVB sheet to glass.
Accordingly, further improved methods are needed to enhance the blocking resistance of PVB sheet, without adversely affecting optical clarity of laminates and glass adhesion properties of the resulting PVB sheet.
It now surprisingly has been discovered, according to the present invention, that a polymer sheet comprising a fatty acid amide features enhanced antiblock characteristics without adverse optical and adhesion effects. Also, it has been discovered that antiblocking characteristics may be imparted to a surface of a polymer sheet by a method of manufacture comprising incorporating a fatty acid amide with the polymer. Further, the present invention includes a laminated safety glass comprising two sheets of glass with an interlayer polymer sheet disposed therebetween wherein the polymer sheet has a fatty acid amide incorporated therewith.
In particular, according to the present invention, methods are disclosed to enhance the blocking resistance of a polymer sheet comprising plasticized PVB. The methods include incorporating a surface modifying agent with the plasticized polyvinyl butyrate, wherein the surface modifying agent is a fatty acid amide preferably having a hydrocarbon chain having about 12 to about 40 carbon atoms.
By virtue of the present invention, it is now possible to provide PVB sheet in a rolled or stacked form, which is less susceptible to blocking than PVB sheet which does not comprise the surface modifier of the present invention. The methods of the present invention may be applied to any polymer having a suitable glass transition temperature; the methods are particularly useful in conjunction with PVB sheet having a glass transition temperature below about 40xc2x0 C.; the invented methods are especially useful in conjunction with PVB sheet having a low glass transition temperature, e.g., below about 20xc2x0 C. (referred to in the art as xe2x80x9cacousticxe2x80x9d PVB sheet). As a result, the requirement to refrigerate or interleave the PVB sheet during transportation and storage can be reduced or eliminated. In accordance with the present invention, the benefits derived from the enhanced blocking resistance can be achieved without substantially adversely affecting other properties of PVB sheet, e.g., clarity or adhesion to glass.
The present invention also provides processes for manufacturing a polymer sheet comprising plasticized polyvinyl butyral. The processes comprise melt processing plasticized polyvinyl butyral into a sheet and incorporating a surface modifying agent with the polymer sheet. Incorporation of the surface modifying agent may be accomplished by adding the agent directly into a molten batch of the polymer prior to extrusion, or by directly applying the antiblock agent onto plasticized PVB sheet.